Tertiary Kinematics in the Lepontine Dome Geological Society, London
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Geological Society, London, Special Publications Tertiary kinematics in the Lepontine dome O. Merle, P. R. Cobbold and S. Schmid Geological Society, London, Special Publications 1989; v. 45; p. 113-134 doi:10.1144/GSL.SP.1989.045.01.06 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by on 30 May 2007 © 1989 Geological Society of London Tertiary kinematics in the Lepontine dome O. Merle, P. R. Cobbold & S. Schmid SUMMARY: In the Pennine zone of the central (Lepontine) Alps, the mineral-zone boundaries of Tertiary metamorphism and the regional attitudes of foliation and thrust sheets define a dome. We investigate the Tertiary kinematic history of the dome using structural, metamorphic and geochronological data. Thus we distinguish two main ductile deformations, that occurred under differing metamorphic conditions. The first high- temperature deformation (HTD) developed under amphibolite facies conditions. It is responsible for (1) a radiating pattern of stretching lineation trends at the scale of the Lepontine area, and (2) structures indicating overthrusting towards the European fore- land in the lineation direction (i.e. N and NW). The HTD event is attributable to NW-SE Tertiary convergence between Europe and the Adriatic sub-plate. The second retrograde deformation (RD) is responsible for differential uplift of the Pennine zone, with formation of an earlier subdome to the E (Ticino subdome, 25-20 Ma) and a later one to the W (Simplon subdome, 15-10 Ma). In the Ticino subdome, kinematic indicators reveal a horizontal shearing top W. Some major structures such as the Maggia steep zone and the vertical duplication of the Antigorio basement nappe (Bosco-Gurin area) may be related to this westward phase. Dextral transcurrent shearing along the southern steep belt and backthrusting of the central Alps over the southern Alps along the N dipping Insubric mylonite belt are probably coeval with the retrograde deformation recorded in the Ticino subdome. In the Simplon subdome, kinematic indicators reveal a radial motion parallel to the lineation, which plunges towards the SSW in the Verampio window and towards the WSW at the Simplon Line. Serial uplift of Ticino and Simplon subdomes is attributed to continuing dextral transpression at the boundary between Europe and the Adriatic sub-plate. Finally, the back-folding event located at the northern limit of the Pennine zone postdates both HTD and RD deformational events described above. It is part of a later pop-up responsible for uplifting the external crystalline massifs. Introduction Lepontine dome. First, it occupies a critical position within the Pennine zone, which is often In this paper, we use structural, metamorphic considered to represent an originally thinned and geochronological data to study the Tertiary continental margin (or margins) between the kinematics of the Lepontine Alps. Many of the European plate proper to the N, and the data we have taken from the abundant literature southern Alps, belonging to an Adriatic sub- dealing with this area, but some structures plate of African affinity, to the S (Fig. 1). In the (stretch lineation patterns and kinematic indi- Lepontine region, the current boundary be- cators) are presented here for the first time. tween the Pennine zone and the southern Alps The Lepontine region appears as a single is the peri-Adriatic fault system, locally known dome in terms of the mineral zone boundaries as the Tonale, Insubric and Canavese Lines (isograds) of Tertiary metamorphism (Niggli (see Schmid et al. this volume). Second, the 1970) and is usually referred to in this sense as eroded Lepontine dome provides a tectonic the Lepontine dome (Fig. 1). The region is also window through the Pennine zone. Third, the a dome, albeit a composite one, on the basis of dome is also at a critical longitude, between the foliation and thrust sheet attitudes (Fig. 2). The eastern Alps, roofed by Austro-Alpine thrust Pennine zone within the dome shows ductile sheets, and the western Alps, which expose deformations of remarkable intensity and com- mostly Upper Pennine units (Monte-Rosa and plexity, coeval with Tertiary metamorphism of Bernhard sheets, Fig. 1). Finally, in this general amphibolite grade and the later cooling history. area, the peri-Adriatic fault system bends round Thus the Lepontine dome provides an excellent from an EW attitude (Tonale line) to an almost and unusual opportunity for studying deep- N-S attitude (southern Canavese line). In such seated processes of deformation (probably as- a context, it is perhaps not surprising that the sociated with plate convergence), followed by internal structure of the Lepontine dome is shallower processes (associated with uplift). complex. Nevertheless, we feel that an under- There are several reasons for studying the standing of it should be helpful towards a From: COWARD,M. P., DIETRICH, D. & PARK, R. G. (eds), 1989, Alpine Tectonics, II 3 Geological Society Special Publication No. 45, pp. 113-134. II4 O. Merle, P. R. Cobbold & S. Schmid FIG. 1. Simplified geological map of the Lepontine dome. In the Pennine zone, cover rocks (stippled) are interleaved with slices of crystalline basement (Ad: Adula, An: Antigorio, Bd: Bernhard, CI: Cima-lunga, Le: Leventina, Lu: Lucomagno, Ma: Maggia, MI: Monte Leone, Mo: Moncucco, Mr: Monte Rosa, Se: Sesia, Si: Simano, Su: Suretta, Tb: Tambo,). Also shown are Verampio window (Vr), Berisal synform (Be), Glishorn antiform (GI) and Bergell intrusion (Ber). Periadriatic faults (Canavese, Insubric and Tonale Lines) separate the Pennine zone from the southern Alps, which include the Ivrea zone (Iv). Thick dashed line encloses area of amphibolite facies metamorphism of Tertiary age. Cs indicates the line of the cross section shown in Fig. 9. For numbered localities and peaks see key at bottom fight. broader-scale understanding of Alpine struc- summarize what little is known of its pre- tures and kinematics. Tertiary history. In more detail, we discuss Since this contribution aims at regional petrological and geochronological data on the kinematics, we primarily make use of the large Lepontine metamorphic event. This allows us scale patterns of foliations, lineations and shear to refer to metamorphic assemblages when de- indicators at amphibolite grade, or later green- scribing linear and planar fabrics or kinematic schist grade. As a consequence, the reader indicators. Using the patterns of lineations and familiar with the existing literature will find that the associated senses of shear, we finally attempt we do little justice in this paper to the complexity a Tertiary kinematic interpretation of the of local deformation phases, especially the Lepontine dome. The inferred motions are so geometrical style, orientation data and inter- thoroughly three-dimensional, that we are not ference patterns of folding on all scales (such as yet able to use balanced cross-sections, or any discussed in Huber et al. 1980). This short- other method, to calculate total amounts of coming simply results from the fact that we are displacement. not yet able to fully integrate this kind of struc- tural analysis into a regional kinematic history and that we are not aware of any previous The structural dome attempts to do so. We first describe the general structure of the In the Lepontine region, a geological map of Lepontine dome itself, including the thrust sheet the Pennine zone (Fig. 1) looks much like a sequence and the foliation pattern. Then we sliced onion, for several geometrical reasons. Lepontine dome - Tertiary kinematics I 15 FIG. 2. Current attitude of main foliation in western and central parts of the Lepontine dome. Map at left shows smoothed strike and dip of foliation. Steep dips of 60~ to 90~ are shown by black dot centred on strikeline; moderate dips of 30~ to 60~ by tick on downdip side of strikeline; flattish dips of O ~ to 30~ by lack of symbol. Numbered localities are as in Fig. 1: Biasca (Bi), Bellinzona (Be), Locarno (Lo), Bosco-Gurin (Bg), Domodossola (Do), Brig (Br) and Ilanz (I1). Data from Schmidt & Preiswek (1908), Knup (1958), Milnes (1974b), Steck (1984), Mancktelow (1985), supplemented by the authors' own measurements. Block diagram at right was drawn by computer using true perspective. Platelets are tangent to foliation as estimated at sea level, after regional averaging in areas 1 kmx 1 km square and interpolation using multiquadrics (Barbotin and Cobbold, work in progress). First, in analogy with the shells of an onion, the (Leventina, Lucomagno), considered infra- region is composed of sheets of Hercynian Pennine by some authors (e.g. Milnes 1974a). basement, typically a few kilometres thick, The overlying Simano sheet dips gently away to interleaved with thinner sheets of Mesozoic the E and W. Further E, the successively higher sediments (Schmidt & Preiswerk 1908, Argand Adula, Tambo and Suretta sheets dip eastward 1911, Staub 1924, Amstutz 1971, Milnes 1974a). by up to 25 ~ on average. They are therefore Early authors described these structures as fold predicted to occur at depths of up to 20 km nappes, but Milnes (1974b) showed that large or so beneath the overlying Austro-Alpine scale folding postdates the emplacement of units which form the eastern boundary of the thrust sheets. Lepontine dome, and are attributed to the Grossly parallel to the sheets themselves (and Adriatic sub-plate. In the SW corner of the to the axial surfaces of major post-nappe iso- region, the uppermost Pennine sheets (Monte- clines) is a composite foliation at amphibolite Rosa and Bernhard units) are separated from grade. We have verified and, where necessary, lower ones by the Simplon Line, which is basi- supplemented the foliation attitudes measured cally a SW dipping normal fault (Mancktelow by Schmidt & Preiswerk (1908), Knup (1958), 1985).